This invention relates to removable closure apparatus for hollow columnar members. More particularly, the invention relates to removable bottom closure apparatus for open-ended tubular piles, pile guides and jacket columns and conductor pipes used in offshore platform structures.
In the initial stages of some offshore platform installations, the structure is required to be buoyant in order to permit controlled launching and erecting of the platform into position for placement on the ocean floor. Typically, buoyancy has been provided by closing the ends of jacket legs that form the support for the platform and through which piles are to be driven. Alternatively, buoyancy for the platform structure can be provided by outfitting the structure with shirt sleeves which are closed at the ends. A further alternative is to insert tubular piles within the jacket legs or within pile guides and close the ends. In any of the foregoing approaches to providing an offshore structure with buoyancy, it has been conventional practice in the art to close the ends of a jacket column, pile guide or open-ended pile with some type of an end closure.
As a platform is erected to assume the proper upright position for permanent installation on the ocean floor, the buoyancy provided by the closures is varied by selectively flooding closed chambers. It thus becomes necessary to remove the closures in order to permit the driving of pilings into the ocean floor to secure the structure in place or for insertion of drill pipe in conductor pipes.
One approach in the prior art includes a platform structure with buoyancy by welding a closure plate in the lower end of a jacket column. Removal of the closure plate is by driving a pile against the closure plate with sufficient force to rupture weld that holds a closure plate in position. This solution, however, has not been found to be satisfactory. Often times a portion of the weld or closure plate will remain attached to the inside surface of the jacket column presenting a restriction to the driving of the pile. Also, closure plates removed in this manner has been found to be susceptible to wedging within the jacket column, making it further difficult to drive a pile through the column.
Another approach to the problem of providing an effective removable lower closure member for a jacket column on an offshore platform is disclosed in U.S. Pat. No. 3,613,381. A lower closure member is welded at its periphery to the wall of the jacket column. It is removable through the upper end of the jacket column by breaking the weld away from the surrounding wall and pulled the fragments upwardly through the jacket column on the end of a pull line. The closure member is in the form of a truncated cone welded at its periphery to the wall of the jacket column. A tearing arm is rigidly secured to the closure member near its periphery and extending upwardly to a point above the closure member. The arm is connected to a pull line extending through the column to the upper end thereof. A length of heavy chain extends around the periphery of the closure member on its lower side and is welded at spaced points to the closure member with one end of the chain being secured to the arm. The chain forms a tear strip which, upon the application of an upward strain of adequate magnitude to the pull line, tears the weld that holds the closure member to the wall of the jacket column.
A residual rim of steel or weld metal is left on the inner surface of the jacket and can present a substantial restriction to the driving of a pile therethrough.
The prior art also includes a removable closure for the lower end of a jacket column disclosed in U.S. Pat. No. 3,577,737. A watertight plug assembly utilizes a compressible rubber member for engaging the inner surface of the wall of the jacket column to seal off the lower end of the column. In combination with the rubber member, the plug assembly includes a mechanism for applying pressure to the top and bottom sides of the rubber member causing it to expand laterally and engage the inner surface of the jacket column wall. To remove the plug assembly, the pressure applied to the top and bottom of the rubber member is relieved allowing the rubber member to resume its original shape and to disengage the jacket column wall. This closure, however, has the obvious disadvantage of requiring mechanical locking devices and moving parts that can malfunction.
The prior art also includes a removable closure using a rupturable diaphragm of reinforced elastic material and is removable in the sense that the elastic diaphragm can be ruptured by driving a pile therethrough.